Pie chart / stacking

[luucvanderzee]

To stay true to the 'tidy data' principles, and the '1 row -> 1 mark' philosophy, I suggest we use implement pie charts as 'one slice of the pie per row', with one column representing the size of the slice, and one column representing the category of the slice. Stacked bars would work according to the same principle, since a pie chart is essentially a stacked bar chart in a polar coordinate system.

However, to stack bars we need to have some sort of cumulative sum data transformation, which we could use in combination with the nextRow/prevRow that is available in the vgg-map's default slot scope. So I guess our to do list here is

• Finish the categorical color scale issue (Categorical color scale #61)
• Add a cumsum data transformation
• Create a high-level pie chart component
• Create a high-level stacked bar chart component?